The present invention relates to dynamoelectric machines and, more particularly, to techniques for insulating the windings of dynamoelectric machines.
Dynamoelectric machines of medium to large size employ metallic conductor bars inserted into slots in metallic rotors or stators. Some such conductor bars operate in a dirty environment which can provide an unwanted conductive path between adjacent metallic conductor bars when they are operated at substantially different voltages. Although the present invention is not limited to such applications, the problem of maintaining insulation integrity becomes particularly severe in the armatures of DC motors where environmental contaminants, as well as conductive carbon dust produced by the unavoidable wear of carbon brushes may be deposited on parts within the DC motor, including the conductor bars and their insulation.
Medium to large DC motors may employ sets of two or more conductor bars in a single slot in a metallic rotor. Each of the conductor bars in a slot must be insulated from all other conductor bars in the slot. The bars must also be insulated from electrical contact with the metallic rotor. As the bars emerge from the end of the slot, they enter an end turn region wherein they are each bent at an angle with respect to the rotor slot. At the outboard end of the end turn region, the bars are stripped to form lead ends for electrical connection to the remainder of the electrical circuit. The bars diverge slightly in the end turn region, particularly in the vicinity of the lead ends, and thus create a crevice therebetween which is conductive to the collection of carbon and other conductive dust which may become mixed with water or oil to form an adhering conductive layer on and between the conductor bars.
In order to improve the insulation in the vicinity of the transition from the end turn region to the lead end region, it is conventional to wrap this transition of each conductor bar with a woven glass fiber tape and to vacuum impregnate the glass fiber tape with an insulating resin forming the bundle making up the several conductors and inserting them into a single slot in the rotor. The glass fiber tape is conventionally applied by hand using relatively skilled labor. This taping operation represents a significant cost in motor manufacturing and rebuilding.
One technique for insulating conductors includes an insulating tube which may be slipped over the end of the conductor and may then be impregnated with a resin. The use of such insulating tubing for insulating electrical conductors is disclosed in U.S. Pat. Nos. 702,725 and 424,575, among others. Conventional tubing is formed by braiding a large number of individual strands to form a tube. However, conventional braided fabric tubing tends to fray at its cut end unless coated with an unacceptably thick coating of a retaining resin. Fraying at the cut end of the braided fabric tube is quite well illustrated in FIGS. 1 and 2 of the above-referenced '725 patent. Such frayed ends provide a favorable environment for the deposition of conductive contaminants and electrical bridging between adjacent conductors, thereby seriously degrading the insulation provided by the tube.
Braided tubing is relatively expensive because of the large factory space and labor required for its production. Insulating tubing may be produced by the more economical knitting process. Although a knitted tubing resists fraying, the cut end therof generally curves outward to produce a bell-mouth effect which provides a hospitable environment for the deposition of conductive contaminants between the cut end adjacent conductor bars. In addition, the thread gauge, stitch spacing and row spacing in conventional knitted tubing provides a rough surface which is far from optimum in its ability to resist the deposition of a contaminant layer thereon. For these reasons, conventional knitted tubing is not satisfactory for insulating the recited areas of conductor bars on DC motor rotors.